The present invention relates to a beam scanning device for simultaneously scanning and exposing a single photosensitive drum with a plurality of laser beams, to form a single electro-static latent image on the photosensitive drum, and an image forming apparatus such as a digital copying machine or a laser printer using the beam scanning device.
In recent years, various digital copying machines have been developed in which image formation is performed by scanning and exposing with a laser beam and electronic photographing processing.
More recently, in order to obtain a higher image forming speed, developments have been made to a digital copying machine adopting a multi-beam method in which a plurality of laser beams are generated and scanning is simultaneously carried out for a plurality of scanning lines with use of a plurality of beams, in order to improve the image formation speed.
This kind of digital copying machine which adopts such a multi-beam method comprises a plurality of laser oscillators for generating laser beams, a polygon rotation mirror for reflecting the laser beams generated by the plurality of laser oscillators toward a photosensitive drum to scan the photosensitive drum with the laser beams, and an optical unit as a beam scanning device mainly consisting of a collimator lens and an f-.theta. lens.
However, in the structure of a conventional optical unit, it is very difficult to obtain an ideal positional relationship between a plurality of beams on a photosensitive drum (or a surface to be scanned). In order to obtain an ideal positional relationship, respective components as well as assembling thereof require high accuracy, and hence, the cost of the device is increased.
Even if an ideal positional relationship is obtained, the shape of a lens may be changed slightly or the positional relationship between respective components may be changed slightly due to circumferential changes, such as changes change in temperature and humidity or time-based changes. Consequently, the positional relationship between beams changes, and as a result, a high quality image cannot be formed. Therefore, to construct this kind of optical system, it is necessary to adopt a structure and components which are strong against changes as described above. Especially, as for lenses, a glass lens which is strong against circumferential changes and time-based changes is expensive so that the cost of the device is increased.
In the following, defects in an image which are caused when an image is formed with beams whose passing positions are erroneously dislocated will be explained with reference to FIGS. 15A and 15B and FIGS. 16A and 16B.
For example, in case where a character of "T" shown in FIG. 15A is formed, an image as shown in FIG. 15B is formed when a passing position of a beam is erroneously dislocated from a predetermined position. In the example of this figure, the passing position of a beam b is shifted from its predetermined position so that the distance between beams a and b is reduced while the distance between beams b and c is enlarged, among four beams a to d used.
FIG. 16A shows an example of an image in which emission timings of respective beams are not controlled correctly. As is apparent from this figure, the image forming position in the main scanning direction is dislocated so that a longitudinal line cannot be formed straight.
FIG. 16B shows an image in which neither the passing positions of beams nor the emission timings are controlled correctly, defects in an image appear both in the sub-scanning direction and in the main scanning direction.
Thus, when an image is formed in a multi-beam method, beam passing positions in the sub-scanning direction must be controlled to be arranged at predetermined intervals, and the emission timings of respective beams must be controlled so as to align the image forming position in the main scanning direction.